There is today in the petroleum industry a steadily increasing demand for high quality middle distillate products boiling in the range of about 300.degree.-700.degree. F. Such products include for example aviation turbine fuels, diesel fuels, heating oils, solvents and the like. To satisfy the demand for these products, it has recently become desirable to supplement the older refinery procedures of distillation and catalytic cracking with catalytic hydrocracking.
The industrial development of catalytic hydrocracking over the past 15-20 years has been aimed primarily at the production of lower boiling products such as gasoline, and highly active catalysts have been developed for that purpose. These catalysts usually comprise a highly acidic cracking base such as a hydrogen Y zeolite or silica-alumina cogel, upon which is deposited a suitable hydrogenation metal component. In attempting to utilize these catalysts for the conversion of heavy oils boiling above about 700.degree. F to middle distillate products, it was found that selectivity was a major problem. Under hydrocracking conditions sufficiently severe to give economical conversions per pass, a large proportion of the feed was converted to products boiling below about 400.degree. F. Improved yields of middle distillate products could be achieved by operating at lower temperatures, but this entailed the uneconomical alternatives of operating at very low space velocities and/or low conversions per pass. It became apparent that some modification of the strength of the acidic cracking sites would be needed.
Since conventional hydrofining catalysts such as cobalt molybdate supported on alumina were known to display some moderate cracking activity, attempts were made to utilize such catalysts under hydrocracking conditions to obtain a more selective conversion of heavy feeds to middle distillate products. Improved selectivity was obtained, but only by resorting to the uneconomical alternatives of operating at very low space velocities and/or at high temperatures entailing short run lengths. The cracking activity of such catalysts was insufficient to provide a commercially feasible process in situations where maximum middle distillate yields were required.
Various attempts were made to increase the cracking activity of such hydrofining catalysts without sacrificing selectivity. One such attempt is described in my U.S. Pat. No. 3,853,742, in which minor proportions of certain zeolite cracking bases were incorporated into the catalyst. Improved activity was obtained, but at considerable sacrifice of selectivity. Another such attempt is described in U.S. Pat. No. 3,306,843, wherein various proportions of silica gel ranging between about 5 and 70% were incorporated into the catalyst in order to improve cracking activity. Here again, improved activity was obtained only at the expense of drastic losses in selectivity, as indicated by the following data from Example VI of said patent:
______________________________________ Hydrocracking Heavy Gas Oil [Charge: Kuwait Vacuum Gas Oil (20.7.degree. API, 3.2% S, 970 p.p.m. N). Conditions: 800.degree. F., 2,000 p.s.i.g., 1.0 LSHV, and 10,000 s.c.f. H.sub.2 /bbl. Catalyst: 6% Ni, 19% W, 2% F.] ______________________________________ Percentage of Total (SiO.sub.2 + Al.sub.2 O.sub.3): Percent SiO.sub.2 5 10 30 70 Percent Al.sub.2 O.sub.3 95 90 70 30 Activity, percent by vol. off at .degree. F. ASTM: 400 16 23 45 83 675 67 81 92 100 (+) ##STR1## 2.82 1.58 0.91 (1) ______________________________________ (1) Unavailable. Estimated to be approximately 0.20.
At this stage in the development of the art, it appeared that activity and selectivity of hydrocracking catalysts were inherently inversely related to each other; the one could be maximized only by substantially sacrificing the other. The present invention is based upon my discovery of a mode in which silica gel can be incorporated into conventional alumina-based hydrofining catalysts to achieve a substantial increase in overall activity, but with substantially no decrease in selectivity. Insofar as I am aware, in the prior art silica was always incorporated into the catalysts a a homogeneous cogel with the alumina base. According to my invention, silica is heterogeneously dispersed in the alumina base, in the form of a silica-rich, silica-alumina cogel or graft copolymer. The alumina base thus provides a "matrix" in which the finely divided silica-alumina composite is dispersed. In this form, the dispersed silica-alumina provides the desired increase in cracking activity, but apparently some moderating effect of the alumina matrix in close association therewith preserves the original selectivity. The final catalysts, containing a minor proportion of active metal component comprising molybdenum and/or tungsten plus nickel and/or cobalt, not only display a desired increase in activity with no significant loss in selectivity, but are very effective for the hydrodecomposition of organic sulfer and nitrogen compounds in the feed.